Process for the manufacture of oxide layers on aluminum and aluminum alloys and to electrolytes therefor



Patented 4, 1941 PROCESS FOR THE MANUFACTURE OF OXIDE LAYERS N ALUMINUM AND ALUMINUM ALLOYS AND TO ELECTRO- LYTES THEREFOR Joachim Korpiun, Berlin-Grunewald, Germany, assignor, by mesne assignments, to

Shcrka Chemical Co. Inc., Bloomfield, N. L, acorporation of New York No Drawing. Application-December 7, 1936,

- Serial No..114,644. 11, 1935 15 Claims.

This invention relates to' -a process 'for the manufacture of oxide layers 'on aluminum and' aluminum alloys and to electrolytes therefor.

It is known that not all electrolytes are suitable for the electrolytic manufacture of oxide protective layers on aliuninum and aluminum alloys.

Hitherto for technical application only sulphuric acid, oxalic acid and chromic acid with or' without additions of other substances have been proposed. The baths containing free sulphuric acid yield on the aluminum with direct or alternating current colorless to whitish layers, the potential amounting to about 5-25 volts. The hardness of these layers is in general not ve y reat which is seen-from the fact that the oxide layer can be scraped ofl with a knife. When it is desired to obtain very hard layers one was hitherto obliged to employ as electrolyte for the oxidation of the aluminum, bath liquids containing oxalic acid and in that'case to work with high potentials of about'40-60 volts. With this high potential naturally also a high consumption of energy for the manufacture of the oxide layers is necessary. Moreover layers so obtained are more or lessbrown colored which is not always deslredl aluminum objects can be produced with .con-

siderably lower potentials. 'The processof this invention comprises the application of electrolytes which contain hydroxy or 0x0 sulphonic acids or sulphonic acids containing carbonyl groups, as, for example, the carboxylgroup, in particular of aromatic nature, alone or with the addition of small quantities of sulphuric acid or sulphates. In this case it has been shown that the polysulphonic acids, as, for example, the disulphonic acids possess advantages compared with the monosulphonic acids, since in. the case of the former higher current densities can be employed. There are obtained (with the addition' of small quantities of sulphuric acid) in the case of potentials of about 15-30 volts and temperatnres between 15 and 35 C., on; the aluminum, oxide layers which exhibit the hardness of the layers hitherto attainable only in baths containing oxalic acid. For the anodic treatment of the aluminum or its alloys in the electrolytes according to the present invention there can be employed both alternating and also direct current, if desired in combination with one another, but on anodic treatment with direct current thickerand harder layers are obtained. In this case the expenditure of energy necessary for the In Germany December manufacture is only about 40-50% compared with the expenditure of energy which is necessary for It has proved to be suitable to add to the said electrolytes containing sulphonic acids, sulphuric acid in quantities which in general should be smalland advantageously 50 grams of sulphuric acid per litre of electrolyte should not be exceeded, if the above indicated hardness of the oxide layers with the electrolyte composed of sulphonicgacids according to the present invention is required-to be obtained. With good results ingeneral sulphuric acid additionsare employed which correspond to about 1-10% by weight of the quantity of sulphonic acid.

For the technical utilisation of the said sulphonic acids it is extremely important that the electrolytes for the purpose of obtaining very hard oxide layers according to the invention should contain relatively small quantities of sulphuric acid. It is to be recommended therefore in manufacture on a commercial scale to employ crude sulphonic acids which are available containing an excess of sulphuric acid the removal of which from the sulphonation product is not necessary, that is to say thereforethat for the manufacture of hard oxide layers the crude sulphonation mixture can be employed insofar as its content of free sulphuric acid falls within the stated limits.

It is also possible to carry out the oxidation of the aluminum or its alloys in baths which contain sulphonic acids according to the. invention and the content of sulphuric acid of which essentially exceeds the above stated limits. The oxide layers obtained from such electrolytes are certainly capable of commercial application but they do not exhibit the great hardness of the layers whichare obtained with a relatively small content of sulphuric acid.

The possibility of producing by electrolytic oxidation of the aluminum in electrolyteswhich consist of solutions of sulphonic acids containing hydroxy or 0x0 or carboxyl groups, very hard oxide layers on aluminum was the less to be expected since researches by others on the electrolytic oxidation of aluminum in solutions which contain sulphonic acids, particularly of aromatic hydrocarbons, have shown that these oxide layers are soft and capable of being polished.

It has further been ascertained that in carrying out the oxidation on a, commercial scale in the presence of sulphuric acid long working may easily cause change of the sulphuric acid content whereby an undesired alteration of the method of working of the bath, is caused. This alteration usually proceeds in such a manner that the sulphuric acid content of the bath decreases. In order to maintain the bath permanently of the same composition therefore from time to time analytical controls of the bath composition are necessary. This is, however, not desirable in practice.

According to a further feature of this invention this disadvantage can be removed when the sulphuric acid is advantageously replaced by sulphates, in which case within wide limits a change 'of the concentration of the added sulphate does not affect the method of working of the bath.

20.f-'There then takes place to some extent a bufier action which allows of an undisturbed operation of the bath without particular analytical controls over long periods of time.

From this it is seen that the sulphate ions do 5. not alone change the method of operation of an oxidation bath containing sulphonic acids, for otherwise the effect of the additions of free sul-' phuric acid should be the same as of the addition of sulphates. It must rather be assumed that 30 by the addition of sulphates to the sulphonic acids particularly stable conditions are produced in the bath which are very desirable from a commercial point of view.

7 As addition in accordance with the invention 35- all sulphates can be employed which are soluble in the solutions of the sulphonic acids and do not enter into any-reactions with these which alter the composition. By selection of the kation of the'added sulphates the properties of the lay-. .40 ers obtained in the'anodic oxidation can be influenced to a far reaching extent. Thus, for

example, the sulphates of the alkali metals give 1 .lifi sulphonic acids certain quantities of sulphuric acid are always contained, the removal of which would render necessary particular'working operations, this I excess sulphuric acid can be em-- ployed for the formation of the desired sulphate.

' 60-=This takes place, for example, in such a manner thateither to the sulphonic acids still containing free sulphuric acid the sulphonic acid metal salts of the metals coming into question are added or the sulphuric acid is neutralised by' the (ii-hydroxides, carbonates or other suitable com- Pounds of the 'said metals. On account of the considerably greater. acid strength 'of the sulphuric acid this is thereby first neutralised.

, If on the other hand the process is carried out 70 in such a manner that suitable metal sulphonates are added to the crude sulphonic acid then there are produced the corresponding metal sulphates with liberation of the sulphonic acid.

The following examples illustrate the inven- I 75 tion:

of the density 1.25. The oxidation itself takes Example 1 As electrolyte for the oxidation of aluminum is employed a solution of cresolsulphonic acid place with an alternating current of 40-50 volts at a bath temperature of about 25 C. Light c01- ored layers of medium hardness are obtained.

. Example 2 The oxidation of the aluminum or its alloys .takes place in the solution of a crude phenol sulphonic acid (sulphonation mixture), whichstill contains about 5% by weight of free sulphuric acid. The density of the acid solution amounts to about 1.2. With the application of alternating current and a potential of 26 volts in this electrolyte very hard oxide layers are obtained.

Ei'rample 3 a As electrolyte in the oxidation serves a solution of 400 grams of thymol sulphonic acid and a 10 grams of sulphuric acid per litre. By the application'of direct current and a potential of volts on aluminum and itsalloys extremely hard 25 oxide layers are obtained. The layers exhibit a greyish tinge. By oxidising with this bath with direct current and a potential of about 20 volts there are obtained colorless layers which are however, somewhat softer than the layers ob- 30 tained at 25 volts, but they still surpass in hardness those obtained according to Example 2.

Example 4 As electrolyte for the oxidation of aluminum is employed m-cresoldisulphonicacid (cryst.) in a quantity of 300 grams per litre of bath liquid, which'in addition contains 2% of sulphuric acid. Working is conducted with potentials of 15-40 volts according to the alloy employed. 40

Instead of the sulphuric acid addition ther can also be employed an addition of 5% magnesium sulphate; The layers thus obtained on the aluminum are somewhat harder than by the application ofthe monosulphonic acid. I

Example 5 An electrolyte suitable for the electrolytic oxidation of aluminum and the like is obtained by treating a solution of 350 grams of pure thymol sulphonic acid per litre with 90 grams of man ganese sulphate. The addition of,sulphate can be increased up to 180 grams without the layers I obtained in the oxidation being essentially difierent. With higher content of manganese sulphate g on the other,hand with certain aluminum alloys a slow reduction ofth'e wearing capacity of the layers takes place.

Example 6 In a solution of commercial phenolsulphonic acid which contains per litre 250 grams of phenol sulphonic acid and 24 grams of sulphuric acid, the sulphuric acid is neutralised with 25 grams of magnesium carbonate and the, sulphate content of the solution is increased by thefurther addition of 50 grams of magnesium sulphate. With such an electrolyte are obtained commercially valuable oxide layers on light metal which are of practically the same quality when the mag- 0 nesium sulphate content is further increased to 100 grams. 1

Example 7 In a solution which contains'pcr litre 400 grams of cresolsulphonic acid and38 grams of sulphuric acid, by the addition of 160 grams of the chromic salt of cresolsulphonic acid the free sulphuric acid is converted into chromic sulphate. After the addition of a further 20-80 grams of chromic sulphate a bath is obtained the method of working of which in the anodic oxidation is practically independent of the sulphate content.

The oxide layers thus obtained can in the known manner be colored, consolidated and impregnated.

Instead of the above mentioned acids also other monoor pclysulphonic acids can be employed according to the invention, as, for example, naphthol sulphonic acids, vanthraquinone sulphonic acids, sulphobenzoic acids and others. As particularly advantageous have proved, however, the

is intended to cover an amount which is a fraction of the sulphonic acid compound present, usually less than 10% of said compound.

What I claim is: s

1. A method for the electrolytic formation of oxide protective films on aluminum and alloys thereof which comprises subjecting the same to anodic oxidation in a bath consisting essentially of anaqueous solution of a sulphonic acid of an aromatic compound containing at least one sub- .stitu'ent taken from the class consisting of hydroxy, oxo and carboxyl.

2. A method for the electrolytic formation of oxide protective films on aluminum and alloys thereof which comprises subjecting the same to anodic oxidation in a .bath consisting essentially of an aqueous solution of a sulphonic acid of an aromatic compound containing at least one substituenttaken from the class consisting of hydroxy, oxo and carboxyl, said acid having a dissociation constant greater than 1x 10* to 1 x 10 3. A method for the electrolytic formation of oxide protective films on aluminum and alloys thereof which comprises subjecting the same to anodic oxidation in a bath consisting essentially of an aqueous solution of a phenol sulphonic acid.

4. A method for the electrolytic isrmation of oxide protective films on aluminum and alloys thereof which comprises subjecting the same to anodic oxidation in a bath consisting essentially of an aqueous solution of a phenol sulphonic acid taken from the class consisting of phenol,

cresols and thymol,

5. A method for the electrolytic formation of oxide protective films on aluminum and alloys thereof which comprises subjecting the same to anodic oxidation in a bath consisting essentialiy= of an aqueous solution of a disulphonic acid of an aromatic hydroxy compound.

6. A method for the electrolytic formation of the class consisting of Mg, I t 10. An electrolyte for the anodic oxidation of oxide protective films on aluminum and alloys thereof which comprises subjecting the same to anodic oxidation in a bath consisting essentially of an aqueous solution of a cresol disulphonic acid.

'7. A method for the electrolytic formation of oxide protective films on aluminum and alloys thereof which comprises subjecting the same to anodic oxidation in a bath consisting essentially of an aqueous solution of a sulphonic acid of an aromatic compound containing at least one substituent taken from the class consisting of hydroxy, oxo and carboxyl, said bath containing a minor amount of a compound taken from the class of sulphuric acid, alkaliv metal sulphates and sulphate's of polyvalent kations.

8. A method for theelectrolytic formation of oxide protective films On aluminum and alloys thereof which comprises subjecting the same to anodic oxidation in a bath consisting essentially of an aqueous solution of a sulphonic acid of an aromatic compound containing at least one substituent taken from the class consisting of hydroxy, oxo and carboxyl, said bath containing an amount of sulphuric acid 'up to 10% by weight of said sulphonic acid.

9. A method for the electrolytic formation of oxide'protective films' on aluminum and alloys thereof which comprises subjecting the same to anodic oxidation in a bath consisting essentially of an aqueous solution of a sulphonic acid of "an aromatic compound containing at least one substituent taken from the class consisting of hydroxy, oxo and carboxyl, said bath containing a minor amount of a sulphate taken from Mn, Cr and Al.

' aluminum and alloys thereof consisting essentially of an aqueous solution of a sulphonic acid of an aromatic compound containing at least one substituent taken from the class consisting of hydroxy, oxo and carboxy.

11. An electrolyte for the anodic oxidation of aluminum and alloys thereof consisting essentially of an aqueous solution of a phenol sulphonic acid.

12. An electrolyte for the anodic oxidation of aluminum and alloys thereof consisting essentially of an aqueous solution of a disulphonic acid of an aromatic hydroxy compound.

13. An electrolyte for the anodic oxidation of aluminum and alloys thereof consisting'essentially of an aqueous solution of a phenolsulphonic acid taken from the class consisting of phenol, cresols and thymol. 14. An electrolyte for the anodic oxidation of aluminum and alloys thereof consisting essentially of an aqueous solution of a cresolidisuh phonic acid.

15. An electrolyte for the anodic oxidation of aluminum and alloys thereof consisting essentially of an aqueous solution of a sulphonic acid of an aromatic compound containing at least one substituent taken from the class consisting of hydroxy, oxo and carboxy, said solution containing a minor amount of a compound taken from the class of sulphuric acid, alkali metal sulphates and sulphates of polyvalent kations.

- J OACHIM KORPIUN. 

